/**
 * @file avatar-url.js
 * @description Fetches an avatar picture from a URL the account typed
 * (Settings → Account, `POST /api/me/avatar/url`), so the server — not the
 * browser, which most image hosts' CORS would block — downloads it. A URL the
 * user names makes the SERVER connect somewhere, so it is held to: `https://`
 * with no userinfo, a host whose EVERY resolved address is public (no
 * loopback, private, link-local, CGNAT, multicast or reserved range — the
 * dashboard's own network is never reachable this way), the connection pinned
 * to the vetted address (a second DNS answer cannot rebind it), at most
 * `MAX_REDIRECTS` redirects each re-checked the same way, a timeout, and at
 * most `MAX_AVATAR_BYTES` read. No cookie or credential is ever sent. The
 * bytes then go through `setAvatar`'s own magic-byte check like an upload.
 * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
 */

const dns = require("node:dns");
const https = require("node:https");
const net = require("node:net");
const { MAX_AVATAR_BYTES } = require("./avatars");

const MAX_REDIRECTS = 3;
const TIMEOUT_MS = 10_000;

const fail = (code, message) => Object.assign(new Error(message), { code });

const blocked = new net.BlockList();
for (const [addr, bits] of [
  ["0.0.0.0", 8],
  ["10.0.0.0", 8],
  ["100.64.0.0", 10],
  ["127.0.0.0", 8],
  ["169.254.0.0", 16],
  ["172.16.0.0", 12],
  ["192.0.0.0", 24],
  ["192.0.2.0", 24],
  ["192.88.99.0", 24],
  ["192.168.0.0", 16],
  ["198.18.0.0", 15],
  ["198.51.100.0", 24],
  ["203.0.113.0", 24],
  ["224.0.0.0", 4],
  ["240.0.0.0", 4],
]) {
  blocked.addSubnet(addr, bits, "ipv4");
}
for (const [addr, bits] of [
  ["::", 128],
  ["::1", 128],
  ["64:ff9b::", 96],
  ["64:ff9b:1::", 48],
  ["100::", 64],
  ["2001::", 23],
  ["2001:db8::", 32],
  ["2002::", 16],
  ["fc00::", 7],
  ["fe80::", 10],
  ["ff00::", 8],
]) {
  blocked.addSubnet(addr, bits, "ipv6");
}

/** True only for a globally routable unicast address. */
function isPublicAddress(address) {
  const family = net.isIP(address);
  if (family === 4) return !blocked.check(address, "ipv4");
  if (family !== 6) return false;
  const lower = address.toLowerCase();
  // An IPv4-mapped/compatible address is judged as the IPv4 address it carries.
  const mapped = /^::(?:ffff:)?(\d+\.\d+\.\d+\.\d+)$/.exec(lower);
  if (mapped) return isPublicAddress(mapped[1]);
  if (/^::(?:ffff:)?[0-9a-f]{1,4}:[0-9a-f]{1,4}$/.test(lower)) return false;
  // Only global unicast (2000::/3) is public.
  if (!/^[23][0-9a-f]{0,3}:/.test(lower)) return false;
  return !blocked.check(address, "ipv6");
}

/** The URL as a `URL`, or EBADURL: https only, no userinfo, a real host. */
function parseAvatarUrl(text) {
  let url;
  try {
    url = new URL(String(text ?? "").trim());
  } catch {
    throw fail("EBADURL", "not a valid URL");
  }
  if (url.protocol !== "https:") throw fail("EBADURL", "only an https:// link");
  if (url.username || url.password) throw fail("EBADURL", "a link with a login is refused");
  if (!url.hostname) throw fail("EBADURL", "not a valid URL");
  // Node never calls `lookup` for an IP-literal host, so it is checked here.
  const literal = url.hostname.replace(/^\[|\]$/g, "");
  if (net.isIP(literal) && !isPublicAddress(literal)) {
    throw fail("EPRIVATEURL", "that address is not on the public internet");
  }
  return url;
}

/**
 * `lookup` for `https.request`: resolves every address and refuses the host
 * when ANY of them is not public, then hands over only vetted ones.
 */
function safeLookup(hostname, options, cb) {
  if (typeof options === "function") [cb, options] = [options, {}];
  const literal = hostname.replace(/^\[|\]$/g, "");
  const done = (addrs) => {
    if (!addrs.length || !addrs.every((a) => isPublicAddress(a.address))) {
      return cb(fail("EPRIVATEURL", "that address is not on the public internet"));
    }
    if (options?.all) return cb(null, addrs);
    return cb(null, addrs[0].address, addrs[0].family);
  };
  if (net.isIP(literal)) return done([{ address: literal, family: net.isIP(literal) }]);
  dns.lookup(hostname, { all: true, verbatim: true }, (err, addrs) => {
    if (err) return cb(fail("EFETCH", `could not resolve ${hostname}`));
    done(addrs);
  });
}

function getOnce(url) {
  return new Promise((resolve, reject) => {
    const req = https.get(
      url,
      {
        lookup: safeLookup,
        agent: false,
        timeout: TIMEOUT_MS,
        headers: { Accept: "image/png,image/jpeg,image/gif,image/webp", "User-Agent": "AgentHub" },
      },
      (res) => {
        const status = res.statusCode || 0;
        if (status >= 300 && status < 400 && res.headers.location) {
          res.resume();
          return resolve({ redirect: new URL(res.headers.location, url).href });
        }
        if (status !== 200) {
          res.resume();
          return reject(fail("EFETCH", `the server answered ${status}`));
        }
        if (Number(res.headers["content-length"]) > MAX_AVATAR_BYTES) {
          res.destroy();
          return reject(fail("ETOOBIG", "the image is too large"));
        }
        const chunks = [];
        let size = 0;
        res.on("data", (chunk) => {
          size += chunk.length;
          if (size > MAX_AVATAR_BYTES) {
            res.destroy();
            return reject(fail("ETOOBIG", "the image is too large"));
          }
          chunks.push(chunk);
        });
        res.on("end", () => resolve({ body: Buffer.concat(chunks) }));
        res.on("error", (e) => reject(e.code ? e : fail("EFETCH", e.message)));
      }
    );
    req.on("timeout", () => req.destroy(fail("EFETCH", "the server took too long")));
    req.on("error", (e) =>
      reject(e.code === "EPRIVATEURL" || e.code === "EFETCH" ? e : fail("EFETCH", e.message))
    );
  });
}

/** Downloads the picture at `text`; resolves to its bytes (unchecked — `setAvatar` sniffs them). */
async function fetchAvatarImage(text) {
  let url = parseAvatarUrl(text);
  for (let hop = 0; hop <= MAX_REDIRECTS; hop++) {
    const out = await getOnce(url);
    if (out.body) return out.body;
    url = parseAvatarUrl(out.redirect);
  }
  throw fail("EFETCH", "too many redirects");
}

module.exports = { fetchAvatarImage, isPublicAddress, parseAvatarUrl };